You cannot perform any tasks at the behest of the company prior to your report time. Quite simply, because it is part of the job and therefore must be performed within duty time, or the company are required to adjust report times to include any other tasks performed at their behest.
As my esteemed colleague BE says, I'd go straight to CHIRP with this one. You could also send it direct to the company's CAA Flight Operations Inspector.
Alternatively, since the company are acting quite blatantly illegally, they would not have a leg to stand on if it was raised with them. In other words, any disciplinary action would be a serious breach of contract, as well as ordering crews to break the law.